Hertzsprung-Russell diagram [for Ejnar Hertzsprung Hertzsprung, Ejnar (ī`när hĕrts`spr
..... Click the link for more information. and H. N. Russell Russell, Henry Norris, 1877–1957, American astronomer, b. Oyster Bay, N.Y., grad. Princeton, 1897. In 1902 he went to Cambridge, England, to study. He returned to Princeton in 1905, was professor of astronomy there (1911–27), research professor
..... Click the link for more information. ], graph showing the luminosity of a star as a function of its surface temperature. The luminosity, or absolute magnitude magnitude, in astronomy, measure of the brightness of a star or other celestial object. The stars cataloged by Ptolemy (2d cent. A.D.), all visible with the unaided eye, were ranked on a brightness scale such that the brightest stars were of 1st magnitude and the
..... Click the link for more information. , increases upwards on the vertical axis; the temperature (or some temperature-dependent characteristic such as spectral class spectral class, in astronomy, a classification of the stars by their spectrum and luminosity . In 1885, E. C. Pickering began the first extensive attempt to classify the stars spectroscopically.
..... Click the link for more information. or color) decreases to the right on the horizontal axis. It is found that the majority of stars lie on a diagonal band that extends from hot stars of high luminosity in the upper left corner to cool stars of low luminosity in the lower right corner. This band is called the main sequence. Stars called white dwarfs white dwarf, in astronomy, a type of star that is abnormally faint for its white-hot temperature (see mass-luminosity relation ). Typically, a white dwarf star has the mass of the sun and the radius of the earth but does not emit enough light or other radiation to be
..... Click the link for more information. lie sparsely scattered in the lower left corner. The giant stars—stars of great luminosity and size (see red giant red giant, star that is relatively cool but very luminous because of its great size. All normal stars are expected to pass eventually through a red-giant phase as a consequence of stellar evolution .
..... Click the link for more information. )—form a thick, approximately horizontal band that joins the main sequence near the middle of the diagonal band. Above the giant stars, there is another sparse horizontal band consisting of the supergiant stars. The stars in the lower right corner of the main sequence are frequently called red dwarfs, and the stars between the main sequence and the giant branch are called subgiants. The significance of the H-R diagram is that stars are concentrated in certain distinct regions instead of being distributed at random. This regularity is an indication that definite laws govern stellar structure stellar structure, physical properties of a star and the processes taking place within it. Except for that of the sun, astronomers must draw their conclusions regarding stellar structure on the basis of light and other radiation from stars that are light-years away;
..... Click the link for more information. and stellar evolution stellar evolution, life history of a star , beginning with its condensation out of the interstellar gas (see interstellar matter ) and ending, sometimes catastrophically, when the star has exhausted its nuclear fuel or can no longer adjust itself to a stable
..... Click the link for more information. . In population I regions (see stellar populations stellar populations, two broadly contrasting distributions of star types that are characteristic of different parts of a galaxy . Population I stars are young, recently formed stars, whereas population II stars are old and highly evolved.
..... Click the link for more information. ) like the spiral arms of galaxies or open star clusters, the stars fall almost exclusively on the main sequence. In population II regions like the nuclei of galaxies and globular clusters, the stars are older and have evolved significantly. The most luminous stars have evolved furthest, and an H-R diagram of such a region will show the upper end of the main sequence depopulated and will show a well-developed giant branch. In such a diagram it appears that the main sequence has "burned down" from the top like a candle. Thus, the point at which the main sequence terminates and the giant branch begins is an indication of the age of a star cluster. A modified H-R diagram of the stars in a cluster of unknown distance can be used to determine the absolute magnitude, or luminosity, of the stars. Since the apparent magnitude of a star of given absolute magnitude depends only on the star's distance, the observed apparent magnitude of the stars can be used to calculate the distance to the cluster.

Hertzsprung-Russell diagram

or H-R diagram

Graph in which the absolute magnitudes of stars are plotted against their colours (a measure of their temperatures). Of great importance to theories of stellar evolution, it evolved from charts begun independently in 1911 by the Danish astronomer Ejnar Hertzsprung (1873–1967) and the U.S. astronomer Henry Norris Russell (1877–1957). On the diagram, stars are ranked from bottom to top in order of increasing brightness and from right to left by increasing temperature. Stars tend to cluster in certain parts of the diagram, especially along a diagonal line, called the main sequence, which is the locus of hydrogen-burning stars of different masses.

Hertzsprung-Russell diagram [′hert·spru̇ŋ ′rəs·əl ′dī·ə‚gram]

(astronomy)

A plot showing the relation between the luminosity and surface temperature of stars; other related quantities frequently used in plotting this diagram are the absolute magnitude for luminosity, and spectral type or color index for the surface temperatures. Abbreviated H-R diagram. Also known as Russell diagram.

Hertzsprung-Russell diagram (redirected from Hertzsprung-Russell classifications)

Hertzsprung-Russell diagram

Hertzsprung-Russell diagram
(redirected from Hertzsprung-Russell classifications)